1. Field of the Invention
This invention relates to a Gauss type rear focus lens in which a rearward group is moved toward the object side along the optic axis to thereby accomplish focusing.
2. Related Background Art
Gauss type lenses have heretofore been utilized widely from the standard to the long focus, but if an attempt is made to obtain a lens of long focus and great relative aperture, it becomes considerably large in size and heavy in weight. Moreover, in the total axial movement of the conventional focusing type, the heavy entire lens system must be moved and therefore, it has been difficult to accomplish quick focusing.
To solve such problems, Gauss type lenses of two-group construction which comprise a forward group and a rearward group each having a positive refractive power and in which the rearward group is moved toward the object side along the optic axis to thereby accomplish focusing are proposed by Japanese Laid-Open Patent Application No. 56-114918, U.S. Pat. No. 4,466,711 and Japanese Laid-Open Patent Application No. 59-143116.
However, these Gauss type lenses are generally suited for the quasi-wide angle to the standard lenses, and the attempt to utilize them as long focus lenses encounters the following problems.
In Japanese laid-Open Patent Application No. 56-114918, the amount of movement of the rearward group which is the focusing group is great during focusing and unless the air space between the forward group and the rearward group is secured sufficiently, it becomes impossible to accomplish sufficient short distance focusing. In U.S. Pat. No. 4,466,711, the lens diameter of the rearward group which is the focusing group is large and therefore, the focusing group becomes heavy, and this is not preferable. In Japanese Laid-Open Patent Application No. 59-143116, the amount of movement of the rearward group which is the focusing group is great and it becomes impossible to accomplish sufficient short distance focusing. Also, the lens diameter of the focusing group is large and therefore, the focusing group becomes heavy and moreover, a great spacing must be secured between the forward group and the rearward group, and this leads to the inconvenience that the compactness of the lens is spoiled.
Particularly recently, with the advance of the auto focus, it has been strongly desired to achieve a light weight of the focusing group in order that the load applied to a drive motor for focusing by the amount of movement and weight of the focusing group may be small and quick focusing may be accomplished.